Method of processing ceramic type ferroelectric materials



United States Patent 3 265,787 METHOD OF PROC ESSING CERAMIC TYPEFERROELECTRIC MATERIALS Ronald J. Brandmayr, Neptune, Arthur E. Brown,Red Bank, and Sam Di Vita and Robert J. Fischer, West Long Branch, N.J.,assignors to the United States of America as represented by theSecretary of the Army No Drawing. Filed Feb. 6, 1963, Ser. No. 256,771

6 Claims. (Cl. 264-56) The invention described herein may bemanufactured and use by or for the Government for governmental purposeswithout the payment of any royalty thereon.

This invention relates in general, to a method of processing ceramictype ferroelectric materials, and in parti'cular, to a method ofprocessing barium titanate.

Barium titanate and other ceramic type ferroelectric materials such ascadmium niobate (Cd Nb O3) lead titanate (PbTiO lead zirconate (PbZrO ormixtures of these materials are important in that their dielectricconstant is about one hundred times higher than that of conventionaldielectric materials provided their compositional and structural purityare high. The value of crystalline barium titanate bodies, for instance,as used in computer elements, ferroelectric amplifiers, memory devices,capacitors, I.F. transformers, etc., depends on the composition,crystalline structure, and grain size of the barium titanate orferroelectric material. This purity will, in turn, depend on the purityof the raw materials used in manufacturing the barium titanate compoundsand in the particular methods of manufacture which determine theparticle size, homogeneity, and the structural purity of the crystals.US. Patent No. 2,990,602 issued July 4, 1961, to R. J. Brandmayr et a1.discloses the hot pressing of ceramic type ferroelectric materials insuch a manner as to produce non-reduced pellets of extremely fine andhomogeneous grain size and excellent dielectric properties. According tothe patent, a prepressed pellet of ferroelectric material is completelyimbedded in a casing of pressed zirconia powder and then hot pressed. Adisadvantage of the method described in the above patent is that thehigh temperature required for hot pressing causes grain growth in theceramic type ferroelectric material. A further disadvantage is that theresulting ceramic body exhibits too great a change in dielectricconstant over a broad temperature range.

An object of this invention is to process ceramic type ferroelectricmaterials, particularly barium titanate, so

that a dense body having a very small average grain size is obtainedwith greatly restricted grain growth. Another object is to prepare apure ceramic type ferroelectric body having no observable Curie pointand an extremely small change in dielectric constant with temperatureover a broad temperature range. A further object is to reduce thetemperature at which ceramic type ferroelectric materials can be hotpressed.

It has now been found that the foregoing objects can be attained by amethod of processing involving placing the ceramic type ferroelectricmaterial in a chamber in a metallic tube, swaging the metallic tube downover the ferroelectric material to precompact the material to arelatively high density, and then maturing the ferroelectric material byfiring or hot pressing.

Swaging is a special application of forging. It is basically a series ofvery rapid light blows, delivered within a machine consisting of aself-contained frame, rotary spindle, dies, hammers, cage assembly, andsuitable motor drive. The spindle is the driven member, and houses theforming die and the two hardened hammers. This assembly rotates insideof the roll cage assembly, which contains hardened steel rollers. Thehead ring, which is pressed into the frame, forms an outer racewayBaTi(C O )2.4H O at 600 C. for 6 hours.

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for the rollers. As the hammers pass under two opposing rollers, thereis no clearance between the die segments. As the hammers pass out fromunder the rollers, centrifugal force causes the hammers, and diesegments, to be thrown outward, separating the die segments. This allowsthe work to be inserted further into the die, ready for the next forgingstroke. Each forging stroke then results in a flow of material and areduction in the crosssection of the piece being forged.

We have found that the swaging process can be used to precompact ceramictype'ferroelectric materials to a relatively high density. After hotpressing the swaged ferroelectric materials, dense bodies are obtainedhaving a very small average grain size with greatly restricted graingrowth, no observable Curie point, and an extremely small change indielectric constant with temperature over a broad temperature range.Moreover, the temperature at which the particular ceramic typeferroelectric material is normally hot pressed can be reduced.

Some preferred embodiments of the invention are illustrated in thefollowing examples.

Example I Barium titanate powder is first obtained by calcining Theaverage particle size of the barium titanate powder obtained is about0.1 micron after calcining. The barium titanate powder is then placed ina tube of 16 gauge, 1 /2 inches in diameter, 0.050 inch wall thickness,SAE #1020 steel tubing one end of which has been sealed with a castaluminum plug. The barium titanate powder is then tamped down and theopen end of the tube sealed with another aluminum plug such that thepowder is tightly packed between the two aluminum plugs. 300 grams ofbarium titanate powder are used. The aluminum plugs are held in place bypunching indentations into the steel tubes. The steel tube containingthe barium titanate powder is then processed through a conventionalswaging machine into which a 1.592 inch diameter die had been inserted.The result was a reduction in diarn- The swaged barium titanate is thenremoved from the steel tube by machining off the steel on a lathe. Thebarium titanate obtained is in the form of a tightly compacted rod. Thebulk density of the swaged barium titanate is found to be 4.6 or about76 percent of the X-ray density for barium titanate. Subsequent kilnfiring of this material revealed that the material could be sintered tovitrification at 2250 F. to 2300 F. to 200 F. lower than the usualrequired firing temperature for barium titanate). Even greater bulkdensities than above indicated can be obtained by evacuating the steelor brass tube containing the barium titanate powder before sealing andswaging. The term vitrification as used herein refers to a thermallymaturing process involving the progressive reduction in porosity of aceramic composition as a result of heat treatment or the processinvolved.

Example 2 A brass tube composed of 66.5 percent copper, 33.0 percentzinc, and 0.5 percent lead is de-aired and then lined by spraying atetrafluoroethylene coating on the inside wall of the tube and baking at700 F. Barium titanate powder obtained as in Example 1 is then placed inthe brass tube and the brass tube then inserted inside an SAE No. 1020steel tube telescope fashion; the steel tube then being swaged down overthe brass tube until a tight seal exists between the two tubes. Thecombination of steel on brass with the barium titanate powder inside thebrass is then swaged through the remaining dies down to 0.625 inch. Thesteel jacket is removed from the brass jacket by machining. The brass isdissolved from the barium titanate compact by immersing the swagedbarium titanate encased in the brass jacket in a solution of 10 percentmercurous nitrate (HgNo and 1 percent nitric acid (HNO The brass havingbeen work hardened by the swaging operation reacts with the mercurysolution causing it to crack open enabling removal of the sample.Further ease in removing the compacted BaTio sample from the metal canbe achieved if the sample is heat treated in vacuum at temperatures onthe order of 600 to 1000 F. after treatment with the mercury solution.The sample is then hot pressed at 1900 F. at 10,000 pounds per squareinch similar to the procedure described in US. Patent No. 2,990,602,except that the die case jacket is composed of Hastalloy X instead ofstainless steel and the plunger rods are composed of Haynes metalceramic LT-Z instead of zirconium oxide (ZrO The Hastalloy X is a hightemperature nickel based alloy and LT-2 is a tungstenaluminum oxide (AlO )-molybdenum (Mo) cermet which enables hot pressing under highertemperatures and pressures than possible using Zr plungers and stainlesssteel die case jackets. Hastalloy X and LT-2 metal ceramic are marketedby the Haynes Stellite Division of Union Carbide Corporationf It isnotted that the temperature of 1900 F. at which this material isvitrified by hot pressing is 100150 F. lower than that normallyrequired. In the above menthod, as an alternative to removing the brassjacket from the barium titanate compact by; solvent means, one can slicediscs from the compact while still encased in the brass tube and thenhot press these discs at 1900 F. at 10,000 pounds per square inchsimilar to the procedure described in US. Patent 2,990,602. In such acase, the brass tube is then removed from the hot pressed bariumtitanate sample and the properties evaluated.

It is determined by means of electron microscopy that the bariumtitanate bodies prepared according to the procedure described in Example2 are characterized by an average grain size of about 0.4 micron. Thisis a smaller average grain size than previously observed in similarmaterial which is hot pressed at temperatures 100 to 150 F. higher thanthe bodies prepared in the manner described herein. It is furtherobserved that the grains in the barium titanate bodies prepared as abovefall within a narrow size range and there is evidence that very littlegrain growth has occurred. There is evidence that grain growth-occurs toa greater extent than above when barium titanate is hot pressed attemperatures in the range of 2000 F. to 2100 F. A further property ofthe barium titanate material prepared according to the above procedureis a very high dielectric constant and no observable Curie point. Thedielectric constant undergoes a maximum change of +4 percent to -15percent in the temperature range 24 C. to C. Barium titanate hot pressedaccording to the procedure outlined in U.S. Patent 2,990,602 usuallyundergoes a change of dielectric constant of +40 to -15 percent in thetemperature range of 24 C. to 140 C.

In carrying out the invention, it is desired to precompact the materialby swaging down to a point approaching the X-ray density of thematerial. The possibility of reaching this density will be limited bythe strength of the metal tubing in which the material is encased.

The foregoing description is to be considered merely as illustrative ofthe invention and not in limitation thereof.

What is claimed is: I

1. The method of processing ceramic type ferroelectric materialscomprising confining a powder of the ferroelectric material in ametallic tube, swaging the metallic tube down over the ferroelectricmaterial to precompact the material to a relatively high density,removing the swaged ferroelectric material from the tube, and thenthermally maturing the ferroelectric material.

2. The method according to claim 1 wherein the ceramic typeferroelectric material processed is barium titanate.

3. The method according to claim 2 wherein the ceramic typeferroelectric material is thermally matured by kiln firing at 2250 F. to2300 F.

4. The method according to claim 2 wherein the ceramic typeferroelectric material is thermally matured by hot pressing at 1900 F.at 10,000 pounds per square inch.

5. The method of processing barium titanate powder comprising confiningthe powder in a steel tube, swaging the steel tube down until thediameter of the steel tube is reduced to 0.625 inch, removing the swagedbarium titanate from the steel tube, and kiln firing the barium titanateat 2250 F. to 2300" F.

6. The method of processing barium titanate powder comprising confiningthe powder in a brass tube that has been de-aired and then inner linedwith a tetrafluoroethylene coating, inserting the brass tube in a steeltube telescope fashion, swaging the steel tube down over the brass tubeuntil a tight seal exists between the two tubes,

" further swaging down the steel tube until the di'lmeter of the steeltube is reduced to 0.625 inch, removin the barium titanate from thebrass tube, and hot pressing the barium titanate at 1900 F. at 10,000pounds per square inch.

References Cited by the Examiner UNITED STATES PATENTS 930,723 8/1909Bolton 264l04 936,403 l0/1909 Bolton 264-104 2,990,602 4/1961 Brandmayr264-332 FOREIGN PATENTS 613,273 1/1961 Canada.

ROBERT F. WHITE, Primary Examiner. R. B. MOFFITT, Assistant Examiner.

1. THE METHOD OF PROCESSING CERAMIC TYPE FERROELECTRIC MATERIALSCOMPRISING CONFINING A POWDER OF THE FERROELECTRIC MATERIAL IN AMETALLIC TUBE,SWAGING THE METALLIC TUBE DOWN OVER THE FERROELECTRICMATERIAL TO PROCOMPACT THE MATERIAL TO A RELATIVELY HIGH DENSITY,REMOVING THE SWAGED FERROELECTRIC MATERIAL FROM THE TUBE, AND THENTHERMALLY MATURING THE FERROELECTRIC MATERIAL.